1. Field of the Invention
This invention relates to a laminate type piezoelectric device used as a driving source of an injector.
2. Description of the Related Art
An injector (fuel injection device) of an internal combustion engine of an automobile, or the like, is constituted in such a fashion that when a valve body of a three-way valve or two-way valve connected to a common rail storing a high-pressure fuel is operated, an open/close state of a fuel passage is changed over to thereby change a pressure condition applied to a nozzle needle, and the nozzle needle is brought into an open state so as to inject fuel.
A solenoid valve has been used ordinarily as a driving source for operating the valve body. Attempts have been made to employ a laminate type piezoelectric device as the driving source so as to finely control the driving source and to precisely control the fuel injection state as described, for example, in Japanese Unexamined Patent Publication (Kokai) No. 11-229993.
However, an injector using the piezoelectric device for the driving source has not yet been put into practical application though proposals have been made as described above.
In the injector, atomization of the fuel must be repeated at an extremely high speed. In cases, atomization is done more than 10,000 times per minute. Therefore, extremely severe conditions are imposed on the piezoelectric device as the driving source when it is used in practice. No piezoelectric device has yet been developed that can be sufficiently used under such severe conditions without inviting cracks, and so forth.
A practical injector must have not only a sufficient driving force but must be small enough to be accommodated in a small accommodation space.
Further, to accommodate the piezoelectric device in the injector, it is effective to accommodate the piezoelectric device into a cylindrical case. When accommodated in this cylindrical space, the piezoelectric device must exhibit excellent dynamic performance (large force generation). When the piezoelectric device is accommodated in the cylindrical accommodation space, the temperature rise resulting from self-exothermy of the piezoelectric device becomes a problem. Therefore, heat radiation performance must also be improved.
In view of the prior art technologies described above, it is therefore a first object of the present invention to provide a piezoelectric device, for an injector, usable for a long time and having excellent durability when applied to an injector.
It is a second object of the present invention to provide a piezoelectric device, for an injector, capable of providing a large force generation and having excellent heat radiation performance when accommodated in a cylindrical accommodation space.
According to the first aspect of the present invention, there is provided a piezoelectric device for an injector, built in an injector and generating a driving force of the injector, characterized in that the piezoelectric device is fabricated by alternately laminating a plurality of piezoelectric layers generating displacement in proportion to an applied voltage and a plurality of internal electrode layers for supplying the applied voltage and, in the piezoelectric device, a relation d(0.1 Ec)/d(1.2 Ec)xe2x89xa70.43 is established, where Ec is coercive electric field which causes the changing of polarizing direction, between an apparent piezoelectric constant d(1.2 Ec) calculated from static elongation when an electric field of 1.2 Ec is applied to the piezoelectric device in the same direction as a polarizing direction while a preset load of 500 N is applied to the piezoelectric device and an apparent piezoelectric constant d(0.1 Ec) calculated from static elongation when an electric field of 0.1 Ec is applied to the piezoelectric device in the same direction as the polarizing direction.
One noteworthy point in the first aspect of the present invention is that the ratio d(0.1 Ec)/d(1.2 Ec) is at least 0.43. When the piezoelectric device generates displacement, there exist a piezoelectric displacement component that immediately starts displacement upon application of a voltage and a 90xc2x0 rotation component that starts displacement with a delay after the application of the voltage, and they together constitute the overall displacement.
The inventors of the present invention have found that a displacement when an electric field 1.2 times the coercive electric field Ec (the details of which will be explained later) is applied is the sum of the piezoelectric displacement component and the 90xc2x0 rotation component described above, and a displacement when an electric field 0.1 times the coercive electric field Ec is applied hardly contains the 90xc2x0 rotation component but almost completely consists of the piezoelectric displacement component.
Therefore, when these apparent piezoelectric constants d are determined, respectively, and their ratio is calculated, the existing ratio of the piezoelectric displacement component contributing to displacement in the piezoelectric device can be determined. In other words, the ratio d(0.1 Ec)/d(1.2 Ec) is the value that replaces the existing ratio of the piezoelectric displacement component when the piezoelectric device undergoes displacement.
Here, the present invention sets the value d(0.1 Ec)/d(1.2 Ec) to at least 0.43. In this way, the present invention can provide a piezoelectric device in which the existing ratio of the piezoelectric displacement component is higher than the 90xc2x0 rotation component. Since the ratio of the 90xc2x0 rotation component is smaller in this case, exothermy of the piezoelectric device due to its repeated displacement can be reduced and, eventually, durability of the piezoelectric device can be improved.
Consequently, the present invention can provide a piezoelectric device that can be used for a long time and has excellent durability when applied to an injector.
According to the second aspect of the present invention, it is more preferable that a relation d(0.1 Ec)/d(1.2 Ec)xe2x89xa70.5 is established between the piezoelectric constant d(1.2 Ec) and the piezoelectric constant d(0.1 Ec).
Next, according to the third aspect of the present invention, there is provided a piezoelectric device for an injector, built into an injector and generating a driving force of the injector, characterized in that the piezoelectric device is fabricated by alternately laminating a plurality of piezoelectric layers generating displacement in proportion to an applied voltage and a plurality of internal electrode layers for supplying the applied voltage; and the piezoelectric device has a change ratio of displacement of 9% or below when a frequency of the applied voltage is changed from 1 Hz to 200 Hz under the state where an AC voltage is applied so that an electric field intensity of 0 to 1.5 kV/mm is generated by a sine wave while a preset load of 500 N is applied to the piezoelectric device.
It is noteworthy in this third aspect that the change ratio of displacement under the condition described above is 9% or below. When the change ratio exceeds 9%, the driving speed of the piezoelectric device cannot be much increased. It is preferable that this change ratio is as small as possible, because, when the change ratio is small, driving can be done at a higher speed. Therefore, according to a fourth aspect of the present invention, it is more preferable that the change ratio is 7% or below.
The change ratio of displacement is expressed by 100xc3x97(Y1-Y200)/Y1 where Y1 is displacement when the frequency of the applied voltage is 1 Hz and Y200 is displacement when the frequency is 200 Hz.
The value of displacement is calculated at 5 seconds after the voltage application.
Next, the function and effect of the third or fourth aspect will be explained.
In the piezoelectric device according to the third aspect of the present invention, the change ratio of displacement described above is 9% or below, or 7% or below. In other words, displacement does not much drop even when the frequency of the applied voltage is increased. Sufficient displacement can be acquired even when the frequency is increased to increase the driving speed. Therefore, the driving speed of the piezoelectric device of this invention can be increased stably. Even when the number of times of fuel injection is 10,000 per minute, the piezoelectric device can repeat displacement (expansion and contraction) with a margin.
Accordingly, the piezoelectric device according to the third or fourth aspect of the present invention can exhibit excellent durability when applied to the injector and can be used for a long time.
According to the fifth aspect of the present invention, there is provided a piezoelectric device for an injector, built into an injector and generating a driving force of the injector, characterized in that the piezoelectric device is fabricated by alternately laminating a plurality of piezoelectric layers generating displacement in proportion to an applied voltage and a plurality of internal electrode layers for supplying the applied voltage; and in the piezoelectric device, displacement increases with the rise of temperature within the range of xe2x88x9240xc2x0 C. to 150xc2x0 C.
It is noteworthy in the fifth aspect that displacement increases with the rise of the temperature within the specific temperature range described above.
Next, the function and effect of the fifth aspect will be explained.
In the injector using the piezoelectric device, a displacement loss resulting from an increase in a fuel leak due to a drop in the fuel viscosity and a displacement loss resulting from a drop in a volume elastic modulus of the fuel occur, so that necessary displacement of the piezoelectric device increases with a temperature rise.
To offset the change of necessary displacement, a control circuit for correcting the change becomes necessary. However, correction by means of a circuit invites the increase of the scale of the control circuit.
In contrast, the piezoelectric device according to the fifth aspect has the feature in that displacement increases with the temperature rise. Therefore, a control circuit for controlling this displacement may well have a relatively simple structure and a relatively small size.
For this reason, the piezoelectric device of the fifth aspect can be easily applied to the injector.
According to the sixth aspect of the present invention, the increase ratio of displacement within the range of temperature of xe2x88x9240xc2x0 C. to 150xc2x0 C. is preferably from 5 to 40%. In this case, the increase of necessary displacement with the temperature rise can easily be compensated for.
The change ratio of displacement is expressed by 100xc3x97(Y150-Yxe2x88x9240)/Yxe2x88x9240 where Yxe2x88x9240 is displacement at xe2x88x9240xc2x0 C. and Y150 is displacement at 150xc2x0 C.
According to the seventh aspect of the present invention, there is provided a piezoelectric device for an injector, built into an injector and generating a driving force of said injector, characterized in that the piezoelectric device is fabricated by alternately laminating a plurality of piezoelectric layers generating displacement in proportion to an applied voltage and a plurality of internal electrode layers for supplying the applied voltage, and the piezoelectric device has a dielectric loss of 8% or below calculated from a P-E hysteresis.
It is noteworthy in this seventh aspect that the piezoelectric device has a dielectric loss of 8% or below determined from a P-E hysteresis. In a graph in which an electric field intensity E is plotted on the abscissa and a charge P, on the ordinate, the P-E hysteresis can be obtained by plotting the trajectory of the value of the charge P when the field intensity is increased up to 1.5 kV/mm and is then lowered (see later-appearing embodiments).
When the dielectric loss determined from this P-E hysteresis exceeds 8%, exothermy becomes so high that the driving speed cannot be increased much. Therefore, according to an eighth aspect of the present invention, it is more preferable that the dielectric loss is 7% or below. Incidentally, the dielectric constant determined from the P-E hysteresis is preferably as small as possible because exothermy can then be suppressed.
Next, the function and effect of the seventh or eighth aspect will be explained.
The piezoelectric device according to the seventh or eighth aspect of the present invention has a dielectric loss of 8% or below, or 7% or below determined from the P-E hysteresis as described above. Therefore, as will be illustrated in the later-appearing embodiments, exothermy of the piezoelectric device can be suppressed even when the piezoelectric device is driven at a high speed, and the durability can be remarkably improved.
In consequence, the piezoelectric device according to the seventh or eighth aspect of the present invention exhibits excellent durability when applied to an injector, and can be used for a long time.
According to the ninth aspect of the present invention, there is provided a piezoelectric device for an injector, built in an injector and generating a driving force of the injector, characterized in that the piezoelectric device is fabricated by alternately laminating a plurality of piezoelectric layers expanding and contracting in proportion to an applied voltage and a plurality of internal electrode layers for supplying the applying voltage, and has an octagonal sectional shape crossing, at right angles, the laminating direction, and the piezoelectric device is accommodated in a cylindrical accommodation space.
It is noteworthy, in the ninth aspect of the present invention, that the piezoelectric device is built and accommodated in the cylindrical accommodation space, and its sectional shape is an octagon or a polygon with a larger number of sides than the octagon.
Since the piezoelectric device has a sectional shape of the octagon or a polygon with a larger number of sides than the octagon, the sectional area of the piezoelectric device, when accommodated in the cylindrical accommodation space, can be increased much more than when the sectional area is a polygonal with lower than eight sides, such as a square or a hexagon, and the accommodation space can be effectively utilized. Therefore, the piezoelectric device of this embodiment can increase the generation force that depends on the sectional area.
When the sectional shape is an octagon or a polygon with a larger number of sides than the octagon, proximity can be increased between the cylindrical accommodation space encircling the piezoelectric device and the piezoelectric device. In other words, portions having a small distance between the piezoelectric device and the cylindrical accommodation space increase or in other words, the space decreases, and heat from the piezoelectric device can be more efficiently transferred to the cylindrical accommodation space. Therefore, when the piezoelectric device generates heat itself, the resulting heat can be easily radiated from the cylindrical accommodation space.
Therefore, the ninth aspect can provide a piezoelectric device, for an injector, that can provide a large force generation and has excellent heat radiation performance when it is accommodated in the cylindrical accommodation space.
According to the tenth aspect of the present invention, there is provided a piezoelectric device for an injector, built in an injector and generating a driving force of the injector, characterized in that the piezoelectric device is fabricated by alternately laminating a plurality of piezoelectric layers expanding and contracting in proportion to an applied voltage and a plurality of internal electrode layers for supplying the applied voltage, at least a part, or the whole, of the sectional shape crossing at right angles the laminating direction is arcuate, and the piezoelectric device is accommodated in a cylindrical accommodation space.
It is noteworthy in the tenth aspect of the present invention that the piezoelectric device is built and accommodated in the cylindrical accommodation space, and at least a part, or the whole, of its sectional shape is arcuate. More concretely, when the sectional shape is a polygon, its corners are rounded to arcs, or a part of the circle is cut into a barrel shape, for example. The radius of curvature of the arcuate shape is preferably close to the radius of curvature of the inner peripheral surface of the cylindrical accommodation space.
Since the piezoelectric device has the sectional shape having the arcuate portions described above, the sectional area when the piezoelectric device is accommodated in the cylindrical accommodation space can be made greater than when the sectional is square or hexagonal. Therefore, the force generation of the piezoelectric device can be increased.
Each arcuate portion can be brought into the state where it is very close to the cylindrical accommodation space encircling the piezoelectric device. Therefore, when the arcuate portions are disposed, the distance can be reduced between the piezoelectric device and the cylindrical accommodation space, and heat transfer can be easily achieved from the piezoelectric device to the cylindrical accommodation space when the piezoelectric device generates heat. In consequence, the temperature rise of the piezoelectric device can be suppressed.
The tenth aspect described above provides the piezoelectric device for an injector that can provide a large generation force and has excellent heat radiation performance when it is accommodated in the cylindrical accommodation space.
According to the eleventh aspect of the present invention, a proximity ratio expressed by (B/A)xc3x97100 (%), where A is the total length of a circumscribed circle of the piezoelectric device and B is the sum of the length of the circumferential portions having a distance of 0.2 mm or below between the circumscribed circle and the piezoelectric device, is preferably larger than 17%. Consequently, heat radiation performance of the piezoelectric device can be further improved, and durability can be improved, too. More preferably, according to the twelfth aspect, the proximity ratio described above is 32% or more, and heat radiation performance can be further improved.
According to the thirteenth aspect of the present invention, at least two side surface portions having a width of 2.5 mm or more are disposed on the side surface parallel to the laminating direction. In this case, the space defined between the side surface flat portions and the inner surface of the cylindrical accommodation space can be effectively utilized, and side surface electrodes for taking out electrodes can be disposed in the piezoelectric device. Incidentally, disposition of the side electrodes becomes difficult when the width of the side surface flat portion is less than 2.5 mm.
According to the fourteenth aspect of the present invention, an insulating film having a thickness of 0.002 to 0.5 mm is preferably formed on at least the side surface of the piezoelectric device in a direction parallel to the laminating direction. In this way, electric insulation can be secured between the piezoelectric device and the injector accommodating the former, and stable control of the piezoelectric device can be obtained. When the thickness of the insulating film is less than 0.002 mm, sufficient insulation performance cannot be obtained in some cases. When the film thickness exceeds 0.5 mm, on the other hand, heat radiation performance of the piezoelectric device drops.
According to the fifteenth aspect of the present invention, a value R2-R1, where R1 is a maximum outer diameter of the piezoelectric device inclusive of the insulating member and R2 is an inner diameter of the cylindrical accommodation space, is preferably 0.5 mm or below. Consequently, heat transfer from the piezoelectric device to the cylindrical accommodation space can be further improved.
According to the sixteenth aspect of the present invention, the insulating film is preferably made of any of a silicone resin, a polyimide resin, an epoxy resin and a fluorocarbon resin. When any of these resins is used, excellent heat resistance capable of withstanding a temperature of 150xc2x0 C. or above, for example, can be obtained in addition to a reliable insulating performance.
According to the seventeenth aspect of the present invention, electrode take-out portions electrically connected to the inner electrode layers are preferably disposed on a distal end face and a rear end face of the piezoelectric device in the laminating direction. In this case, the electrode take-out portions need not be disposed on the side surface of the piezoelectric device in a direction crossing at right angles in the laminating direction, and the structure can be further simplified and rendered compact.
According to the eighteenth aspect of the present invention, either one of the distal end face and the rear end face of the piezoelectric device in the laminating direction is preferably equipped with two electrode take-out portions electrically connected to the inner electrode layers. In this case, electric connection with the piezoelectric device can be established on only one of the end faces. Therefore, not only the structure of the piezoelectric device but also the structure of the arrangement to the injector can be simplified.
According to the nineteenth aspect of the present invention, at least one of the electrode take-out portions is preferably connected electrically to at least one of the inner electrode layers through a through-hole formed in the piezoelectric layers. In this case, the arrangement structure of the electrode take-out portions can be simplified.
According to the twentieth aspect of the present invention, at least one of the electrode take-out portions can take the structure in which it is electrically connected to the side surface electrode disposed on the side surface of the piezoelectric device.